[OE-core] Speed up sdk creation by supporting multicore compressors.

[Randy Witt]

Sorry, I messed up my formatting there, here is are stats that are easier
to read:

Using default compressor(bzip2):
core-image-minimal-1.0-r0: do_populate_sdk: Elapsed time: 133.21 seconds
CPU usage: 12.3%
core-image-sato-1.0-r0: do_populate_sdk: Elapsed time: 356.85 seconds
CPU usage: 11.6%

Using pigz:
core-image-minimal-1.0-r0: do_populate_sdk: Elapsed time: 102.62 seconds
CPU usage: 15.3%
core-image-sato-1.0-r0: do_populate_sdk: Elapsed time: 261.66 seconds
CPU usage: 15.6%
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